1. Field of the Invention
The present invention relates to an exhaust gas cleaning device of an internal combustion engine in which secondary air is supplied into the exhaust system of an internal combustion engine to oxidize and remove unburnt noxious components such as hydrocarbons and carbon monoxide contained in the exhaust gas flowing through the exhaust system.
2. Description of the Prior Art
A conventional exhaust gas cleaning device of the above kind is known as having such a structure that a secondary-air supply system is in communication with the exhaust system of an internal combustion engine and a reed valve device is provided in the secondary-air supply system to be opened and closed by the operation of the internal combustion engine to introduce the secondary air into the exhaust system, whereby unburnt noxious components such as hydrocarbons and carbon monoxide in the exhaust gas are removed. When the above-mentioned exhaust gas cleaning device is mounted on the internal combustion engine, however, cumbersome maintenance is required not only for the device itself but also for the internal combustion engine. For instance, replacement of spark plugs and adjustment of tappets are hindered by such mounting of the cleaning device. Furthermore, complicated construction raises the manufacturing costs.
When the reed valve device is directly mounted on the engine, the device is subject to be affected by the heat of combustion of the engine, whereby deterioration of components of the device due to heat is accelerated to lower their durability. Further, with deteriorated components, the reed in the reed valve device produces increased striking noises as it opens and closes.
When the operation of the engine is stopped, on the other hand, the reed valve device is gradually cooled and the interior of the reed valve chamber often reaches a dew point. Here, the burnt gas flows reversely from the exhaust port into the reed valve chamber. In this case, moisture formed by the combustion of fuel and contained in the burnt gas is coagulated in the reed valve chamber of which the temperature has reached the dew point. As the coagulated water builds up in the flow passage of the secondary air, flow resistance of the secondary air increases to decrease the flow rate of the air. Further, in case an engine has a multiplicity of cylinders, the secondary air may be distributed in unequal amounts into the exhaust systems that are connected to the cylinders.